Third party is minimum legal requirement so anything more than that is by choice and its important to make sure that the policy covers your required number of days abroad (from the UK). A lot of motorhomers also get their breakdown cover though their main insurers otherwise it has to be bought separately and there are often restrictions on age of the motorhome.

It's the motorhome that is generally insured with you, or anyone you want to add, as named driver(s)

As regards companies it may just be a matter of speaking to as many as possible explaining your individual circumstances... this old thread mentions a couple of names https://www.clubmotorhome.co.uk/discussions/4831-insurance-for-australian-travelling-to-europe

Hope that helps and that you'll get some more ideas from others on here